Organopolysiloxanes are typically prepared by the ring-opening equilibration polymerization of cyclic oligosiloxane in the presence of acid or base catalyst. The organopolysiloxanes yielded by this method generally contain low-molecular-weight organosiloxane (having a degree of polymerization (DP) of up to about 25) at a level of from several thousand ppm (parts per million) to several tens of thousands ppm. This can impose rather severe limitations on the application of these organopolysiloxanes. An example of this occurs in the application of silicone products prepared from organopolysiloxane containing large amounts of low-molecular-weight organosiloxane. When such silicone products are used under semi-sealed conditions in the vicinity of a micromotor, relay, or switch, mounted within an electrical or electronic device, electrical discharges convert the low-molecular-weight organosiloxane volatilizing from the silicone product into insulating materials, such as silica and the like. This, in turn, causes such undesirable phenomena as poor contact at relays and switches, micromotor stoppage, and so forth. These phenomena are known as "electrical contact faults." The demand, therefore, exists for organopolysiloxane that contains little low-molecular-weight organosiloxane.
Quite a few methods have been disclosed for the purpose of reducing the level of low-molecular-weight organosiloxane in organopolysiloxanes. Methods such as stripping, solvent extraction, molecular distillation, wiped-film evaporation, and supercritical extraction are already known. However, these methods suffer from a low treatment efficiency when applied to organopolysiloxane containing large amounts of low-molecular-weight organosiloxane, which necessitates multiple treatment passes since a single treatment cannot reduce the low-molecular-weight organosiloxane content to acceptable levels. As a result, in order to reduce the load on these purification processes, the development of a method of organopolysiloxane preparation that provides a minimal content of low-molecular-weight organosiloxane from the very outset is desired.
A method is in fact known for the preparation of organopolysiloxane that contains less low-molecular-weight organosiloxane than organopolysiloxane prepared by equilibration polymerization. This method employs the nonequilibration polymerization of cyclic trisiloxane catalyzed by a lithium compound or pentacoordinate silicon compound (refer, for example, to ACS Polymer Preprints, Volume 10, Number 2, 1361 (1969)). However, even organopolysiloxane prepared by this nonequilibration polymerization methodology contains low-molecular-weight organosiloxane at a level of several hundred ppm to several thousand ppm, with the result that this product is still not fully acceptable for direct application as a starting polymer for the electrical/electronic silicone products discussed above.